In a manufacturing process of paper, online measurement of a basis weight (weight per square meter) and a moisture percentage is very important, and the basis weight is an important control item in terms of both paper quality and conducting business. Conventionally, it is common to use a basis-weight moisture meter (Basis Weight & Moisture Measurement System) to obtain a basis weight of paper from a transmission attenuation amount of beta rays, and paper moisture from an absorbed amount of near infrared rays. Basis weight values measured by a basis-weight moisture meter are known to have the highest reliability.
In order to measure a basis weight, beta rays obtained from a radioactive radiation source such as Kr85 (krypton) or Pm147 (promethium) are used. An attenuation amount of the beta rays is large if the basis weight is large, and the attenuation amount thereof is small if the basis weight is small, and therefore, a transmission amount of the beta rays and the basis weight are almost inversely proportional to each other. In order to obtain an accurate basis weight, a calibration curve representing relation between the transmission amount of the beta rays and the basis weight is used.
Near infrared rays commonly used for measurement of moisture are three types of lights: base light, measured light, and corrected light. A wavelength of the base light is 1.8 μm, and light of this wavelength is not attenuated by moisture. A wavelength of the measured light is 1.9 μm, and light of this wavelength is attenuated by moisture. A wavelength of the corrected light is 2.1 μm, and light of this wavelength is not susceptible to an influence of cellulose. It is possible to obtain an accurate moisture amount by previously examining the relation between an attenuation amount of the measured light due to moisture and a moisture amount as a calibration curve.
Since the beta rays are used in the basis weight measurement using a conventional basis-weight moisture meter, it is necessary to use a radioactive radiation source, such as krypton 85 or promethium 147 as a radiation source. A radioactive radiation source can have harmful effects on a human body. Therefore, in the measurement, a restricted area is provided to keep off a radiation source, a person who frequently works near the radiation source is obligated to carry a film badge, and it is required to check an exposed radiation dose on regular basis. Further, it is required to place a licensed engineer for radiation protection, and great care and specialized knowledge are required for radiation protection.
As a method of avoiding such a problem due to the use of a radioactive radiation source, the present inventors have proposed a method of measuring a basis weight and a moisture of a sheet-like material, such as paper, without using radioactive rays (see Patent Document 1). The proposed method is of measuring a basis weight and a moisture at the same time using a microwave dielectric resonator, and shows that, as an application of an apparatus for online measurement of dielectric anisotropy of a sample such as fiber orientation of paper, it is possible to measure a basis weight and a moisture amount at the same time besides fiber orientation. According to the proposed method, a basis weight is obtained from a resonance frequency shift amount which is a difference between a resonance frequency without a sample and a resonance frequency with a sample provided, and a moisture amount is obtained from a resonance peak level change amount which is a difference between a peak level of the resonance frequency without a sample and a peak level of the resonance frequency with a sample provided.